Hi! I'm an American living in Belgium (Liege). Let me tell you first that each region of Belgium is monolingual (except Brussels and the German region) so in Wallonia they do not speak Dutch and in Flanders they rarely speak French. I would say focus on French because you're more likely to meet Walloons who don't speak English than Flemings. The Flemish are very well versed in English usually and you can go to any of those cities and get by in English. You could survive in Liege and Bastogne in English too but they really appreciate it here if you try a bit of French (where my experience in Flanders is that they just go to English immediately even if you speak some Dutch). In Brussels most people speak French but you can get by on English. Hope that helped!